1. Field of the Invention
This invention relates to an optical recording method, an optical recording medium, an optical recording system, and a method for observing a sample. More particularly, it relates to an optical recording method and an optical recording system making use of a distribution of an optical near field produced from an informative object being irradiated, an optical recording medium having excellent heat resistance, and application of the optical recording method to sample observation.
2. Description of the Related Art
Methods for optically recording information on a recording medium comprising irradiating the recording medium with, for example, a condensed recording light beam from a laser light source to change the reflectance, etc. of the recording medium and recording the change are known. However, not only a laser beam but any other optical systems utilizing light transmitted through gas, etc. cannot be made use of in the region below the diffraction limit of the light. Recording densities smaller than the scale of the diffraction limit can never be achieved, nor can be recorded information of an object smaller than the diffraction limit.
In recent years a so-called optical near field has been attracting attention in this connection. An optical near field can be localized in regions smaller than the wavelength of the light on the surface of an object. Hence, application of an optical near field to high-density recording systems and high-resolution optical microscopes has been proposed.
For example, Motoichi Ohtsu made a report on an optical near field microscope called xe2x80x9cC modexe2x80x9d or xe2x80x9cI modexe2x80x9d in his paper titled xe2x80x9cThe Present and Future Development of Optical Near Field Microscopexe2x80x9d (Kikai no kenkyu, Vol. 49, No. 5 (1997)). The optical near field microscope of C mode picks up evanescent light (optical near field) generated on the surface of a sample being irradiated, by scanning with a fine probe to provide optical data, which are processed to give a three-dimensional image of the surface of the sample. The optical near field microscope of I mode uses a fine probe through which light is transmitted to ooze out an optical near field oozes from its tip. The surface of a sample is scanned with this fine probe to convert the optical near field to scattered light thereby furnishing information data of the sample surface.
However, the above-described optical near field microscopes of C mode and I mode involve the following disadvantages.
(1) Because the probe is brought close to a sample to be observed, it greatly disturbs the electric field around the sample. Therefore, the resulting image is difficult to interpret.
(2) Because a scatter type probe having a very small opening or a very small diameter at the tip is used, the detectable light intensity is small, and the signal/noise ratio (S/N ratio) is not sufficient.
(3) Such processing as integration is necessary for improving the S/N ratio. Considering that scanning with the probe needs some time, the microscope meets difficulty in making an observation on high-speed phenomena or biological cells.
As to optical recording media used in various optical recording methods, it has been keenly demanded to develop a recording medium having high record durability (especially heat resistance) and/or having recorded practically advantageous information.
S. Davy and M. Spajer report in their paper xe2x80x9cNear Field Optics: Snapshot of the field emitted by a nanosource using a photosensitive polymerxe2x80x9d (Appl. Phys. Lett., Vol. 69, No. 22, p. 3306 (1996)) a technique comprising applying an optical near field generated from the tip of a probe to a photosensitive polymer film of an acrylic polymer having an azo dye in the side chain thereof to produce unevenness, which is not optical information recording. This technique is to record the optical near field of a light source. A method for recording the optical near field of an informative object is not disclosed in their report. Nor is given consideration to the thermal stability of the record.
JP-A-61-287791 discloses an optical recording medium making use of a condensational polymer dye, which is characterized by inertness to photo-induced chemical degradation or change of optical properties.
A first object of the present invention is to provide an optical recording method, an optical recording system, an optical recording medium, and a method for observing a sample which are free from the above-described disadvantages (1) to (3) and to provide a technique of optical near field memory for achieving an ultrahigh recording density of several tens of gigabites per square inch and a photolithographic technique applicable to the region below the light diffraction limit.
A second object of the present invention is to provide an optical recording medium having excellent record durability, particularly heat resistance.
The inventors of the present invention have found that, when an informative object (a sample for observation or an object for putting information in) is positioned on or above the surface of a photosensitive material, and that area of the photosensitive material is irradiated with light, photochemical reaction of the photosensitive material takes place more strongly with the optical near field at the part where the irradiated informative object is positioned than with the irradiating light at other irradiated parts. The first object of the invention is accomplished based on this finding.
The inventors have also found that the second object of the invention is accomplished by an optical recording medium prepared by using a polymer containing a photoreactive component capable of photoisomerization and having in the repeating unit thereof at least one group selected from a urethane group, a urea group, an amide group, a carboxyl group and a hydroxyl group. Based on this finding, there are provided a recording medium capable of recording optical information furnished from light for irradiation or an optical near field in a variety of modes, a recording medium useful for holography, a recording medium which contains a specific photoreactive component capable of recording, reading out and erasing information and providing a durable and heat-resistant record, and a recording medium having effectively recorded thereon changes of an informative object with time.
The first object of the invention is accomplished by the following 1st to 4th aspects, and the second object of the invention is achieved by the following 5th to 8th aspects.
The 1st aspect of the invention is an optical recording method comprising constituting a recording layer of a photosensitive material capable of undergoing a storable and detectable photochemical reaction, setting an informative object on or above the recording layer at such a position that an optical near field generated from the informative object may reach the recording layer, irradiating at least the area of the recording layer where the informative object is positioned with light to cause the informative object to generate the optical near field, and recording the distribution of the optical near field on the photosensitive material as a photoreacting quantity of the photosensitive material.
The 2nd aspect is an optical recording method comprising constituting a recording layer of a photosensitive material capable of undergoing a storable and detectable photochemical reaction, setting a mobile or moving informative object on or above the recording layer, irradiating at least the area of the recording layer where the informative object is positioned with light to cause the informative object to generate the optical near field, and recording the distribution of the optical near field on the photosensitive material as a photoreacting quantity of the photosensitive material, the irradiation and recording being repeated two or more times in accordance with the movement of the informative object.
The 3rd aspect provides an optical recording system comprising a recording layer on or above which an informative object is positioned and which is constituted by a photosensitive material capable of undergoing a storable and detectable photochemical reaction, and a light source capable of irradiating at a time at least the area of the recording layer where the informative object is positioned.
The 4th aspect provides a method for observing a sample comprising a recording process comprising constituting a recording layer with a photosensitive material capable of undergoing a storable and detectable photochemical reaction, setting a sample to be observed on or above the recording layer at such a position that the optical near field generated from the sample being irradiated may reach the recording layer, irradiating at least the area of the recording layer where the sample is positioned to cause the sample to generate an optical-near field, and recording the distribution of the optical near field on the photosensitive material as information corresponding to the photoreacting quantity of the photosensitive material, and an observing process comprising observing the recorded information by an observing means selected according to the recording mode.
The 5th aspect resides in an optical recording medium comprising a recording layer for recording optical information with light for irradiation or a generated optical near field, wherein the recording layer comprises a polymer material containing a photoreactive component capable of photoisomerization and having in the repeating unit thereof at least one group selected from the group consisting of a urethane group, a urea group, an amide group, a carboxyl group and a hydroxyl group.
The 6th aspect is an optical recording medium for holography, which has a recording layer comprising a polymer material containing a photoreactive component capable of photoisomerization and having in the repeating unit thereof at least one group selected from the group consisting of a urethane group, a urea group, an amide group, a carboxyl group and a hydroxyl group.
The 7th aspect furnishes an optical recording medium capable of recording, reading out and erasing information which has a recording layer comprising a polymer material containing a photoreactive component which is capable of photoisomerization and the molecular orientation of which can be controlled by light and having in the repeating unit thereof at least one group selected from the group consisting of a urethane group, a urea group, an amide group, a carboxyl group and a hydroxyl group.
The 8th aspect affords an optical recording medium having a recording layer comprising a photosensitive material capable of undergoing a storable and detectable photochemical reaction, the recording layer having recorded thereon at least one of the following pieces of information (1) to (4) in such a mode that a distribution of an optical near field generated from an informative object being irradiated is recorded:
(1) a record of an instantaneous form of a moving informative object;
(2) a record of movement of an informative object which is a fine particle movable by radiant pressure of light;
(3) a record of movement of an informative object which is an autonomically moving living organism; and
(4) a record of changing history of an informative object which shows change with time that can be recorded as optical information.